B2-microglobulin (B2M) is a small peptede that is associated with several cell membrane molecules in adult mice. These associations are necessary for the expression of B2M on cell surfaces. Among these molecules, the H-2K and D transplantation antigens coded for by the mouse major histocompatibility complex, the H-2 complex, are the only B2M-associated malecules that are expressed on the surfaces of nearly all cell types. While B2M is synthesized and expressed by mouse preimplantation embryos, the presence of H-2K/D antigens in embryos has not been confirmed. One aim of this proposal is to use two-dimensional gel electrphoresis to analyze H-2K/D antigens immunoprecipitated with monoclonal antibodies to determine whether H-2 antigens are synthesized during very early development. Additionally, it will be determined whether mRNA transcribed from Class I H-2 genes is present in preimplantation embryos. if experiments indicate the absence of H-2 antigen synthesis in embryos, putative counterparts of the H-2K/D antigens which may be associated with B2M during early mouse development will be analyzed. These molecules are likely to be differentiation antigens that mediate cell-cell interactions during early development. H-2 antigen synthesis increases framatically upon the differentiation of teraocarcinoma cells into cells that resemble embryonic endoderm. the present analysis will examine, therefore, the synthesis and expression of H-2 antigens by newly differentiated embryonic endoderm cells. In addition to studying the molecules that are associated with B2M during mouse embryogenesis, the proposed reaearch will study the regulation of the B2M locus. The molecualr properties of B2M are sililar to the constant regions of immunoglobulin G and suggest that B2M are similar to the constant regions of immune system and the histocompatibility system. This similarity also suggests that the B2M locus may be regulated by allelic exclusion, as are the immunoglobulin genes. this possibility will be investigated using clonal lines derived from mice heterozygous for two allelic forms of B2M. These studies will advance our knowledge of the structure, function, and evolution of molecules associated with mammaliam cell surfaces during embryogenesis and differentiation.